Method and apparatus for drying



A Feb. 2v2, 1966' G. TooBY INVENTOR.

Arranz/frs.

Feb. 22, 1966` G. TOOBY 3,235,971

METHOD AND APPARATUS FOR DRYING Original Filed Nov. 24, 1958 2Sheets-Sheet 2 Gia/mf 720er Mfr/@MXL United States Patent O METHUD ANDAPPARATUS FR DRYING George Tooby, San Marino, Calif., assignor, by mesneassignments, to Hammtronics Systems, Inc., Pasadena,

Calif., a corporation of Delaware I Application Oct. 11, 1961, Ser. No.144,815, which is a continuation of application Ser. No. 776,023, Nov.24, 1958. Divided and this application Mar. 1, 1963, Ser.

7 Claims. (Cl. 34-1) because of the diflculty in supplying the heat ofvaporizav tion of the natural fluids or juices within the material. Inordinary oven drying, the drying progresses from the outer surfacesinward and the energy required to evaporate the moisture on the insidemust be conducted through the outer portion. It has been found that whendiscrete pieces or chunks of materials to be dried, such as fruit,vegetables, or meat, are dried .by conventional hot air processes, thefinal dried product has an outer shell which is hardened more than therest of the product, and the product -as a whole is wrinkled, shrunken,and tough.` Also, materials and food products dried in this mannerrehydrate very slowly and incompletely.

With the advent of radio frequency heating apparatus, a means ofgenerating heat uniformly throughout a nonconducting mass was provided,-and its use resulted in the rapid and uniform drying of many materials.Investigations in radio frequency heating included the dehydration ofpharmaceutical products, foods, and even the cooking of foods. Theseinvestigations indicated that in the preparing of food pieces badelectrical effects including burning resulted when the process wascarried out at the conventional radio frequencies. Accordingly,developmental work was conducted to determine the optimum frequencyrelationships for transferring the heat energy to the food particlesbeing processed. One such investigation is reported in Electronics forOctober 1947, in an article entitled, UHF Heating of Frozen Foods,appearing on pages 85-89.

In general, these electronic methods for processing materials, and foodsin particular, require large power so as to preclude the readyacceptance of these techniques in the home, but the commercialapplications of these techniques for restaurants, bakeries, foodprocessing industries, land the like appear to be quite attractive.

This invention provides a novel and improved method and apparatus fordrying or desiccating food pieces or chunks on a substantially largeorder allowing it to be employed in commercial applications. Thecarrying out of the invention results in an improved palatable, driedproduct which retains its natural color, is of a low density, and evenan improved puffed or expanded product may be produced. The foods to bedried are preferably prepared in pieces or chunks of bite size, andwhich foods may be fruit, Vegetables, meat, and the like having anatural fluid or juice therein responsive to the application of heat.

The drying method comprises subjecting the food pieces toelectromagnetic radiation in the microwave region to supply the heat ofvaporization of the natural fluids or juices therein so as to evaporatethem from within the food pieces. During the time the food pieces areexposed to the electromagnetic radiation, they are also subjected to aheated, dry fluid arranged so as to carry away the fluids or vaporsexpelled from the surface of the food pieces.I This process iscontrolled so that the food pieces are not only free of burns or anyother deleterious electrical effects but also the outer surface isprevented from drying faster than the remainder of the particle, therebyavoiding the formation of a hard outer shell. The temperature of thefluid for conveying the expelled vapors away from the food pieces can becontrollably reduced after -a preselected drying interval to maintain amaximum rate of drying consistent with optimum product quality. Thistemperature change is a function of the moisture content of the foodpieces which, while exposed to electromagnetic radiation, can mostreadily be measured by the power absorbed by the food pieces. If apuffed product is desired, the electromagnetic radiation is increasedevaporating the water within the piece at a higher rate than it candiffuse to the surface so as to expand the food piece and which expandedshape will be retained after the drying procedure is completed.

This novel process is carried out by means of drying apparatuscomprising -a drying cabinet having a metallic casing includingelectrically conducting perforated walls within the casing for definingan electromagnetic cavity therein. The cavity defined in this fashion isprovided with a plurality of low loss dielectric trays or shelvessupported in the cavity, and which shelves are adapted to receive thefood pieces to be dried. The trays are also perforated in the samefashion as the cavity walls. An ultra-high frequency generator forgenerating electromagnetic radiation, preferably of 1,000 megacycles, isassociated with the drying cabinet. The ultra-high frequency generatoris coupled to the cavity so as to permeate the cavity completely withthe electromagnetic radiation. Means are also provided for introducing aheated, dry fluid, such as the ambient air, into the drying cabinet tocarry away expelled fluids and vapors. Means are also provided forexhausting -this vapor laden fluid from the drying cabinet through thecavity, the casing being arranged and constructed to provide a passagefor the conveying fluid therebetween and the perforations or holes inthe walls of the cavity and in the trays provide the means ofcommunication between this passage and the cavity.

These and other features of the present invention may be appreciatedmore fully when considered in the light of the following specificationand drawings, in which:

FIGURE 1 is an elevational View with portions broken away to show theinterior of the drying apparatus and embodying the invention;

FIGURE 2 is a cross-sectional View taken along the line 2*2 of FIG.` l;and

FIGURE 3 is a graphical representation of power loss1 and conveyingfluid temperature during the drying cyc e.

Now referring to the drawings, the apparatus for carrying out the dryingmethod will be explained. The drying apparatus 10 comprises a dryingcabinet 11, including a metallic casing 12 having a cooperating door 13and spaced walls 14, 15, and 16 to define an electromagnetic cavity 17therein. The cavity 17 is arranged to support a plurality of drying`shelves or trays 18. An ultra-high frequency generator 20 is providedto produce the electromagnetic radiation for the cavity 17. A heatingmeans 21 is also provided to heat a vapor conveying fluid to beintroduced into the drying cabinet 11, as well as exhausting means 22for removing this fluid from the dry-ing cabinet.

The drying cabinet 11 is a hollow, thin-walled metallic casing 12compri-sing the top and bottom walls 23 and 24 respectively, and a backwall 25. The front of the casing 12 is enclosed by a cooperating door13, hinged to the bottom wall 24. The door 13 is hinged to openoutwardly from the position shown in FIG. l to thereby allow the dryingtrays 18, food pieces or both to be placed within the drying cabinet 11.The metallic casing 12 includes the cavity Walls 14-16 spaced inwardlyof the walls 23425 to define a plenum passage for a fluid passingbetween these walls. The cavity walls 14-.16 including the inner wall 26for the door 13 are further arranged and constructed to define theelectromagnetic cavity 17 for the drying cabinet 11. The cavity walls14-16 are provided with a plurality. of tray supporting members 27,vertically spaced therein and horizontally aligned to receive andsupport the drying trays 18. The drying trays 18 may be adapted to beslidable on the supports 27 so as to extend outwardly of the door 13 orto be removed entirely to facilitate the placing of the food piecesthereon, as in conventional domestic heating appliances. The dryingtrays 18 may be constructed of a low loss dielectric material ormaterial of high dielectric value such as polyethylene. The drying trays18, however, preferably are constructed of a small mesh screen of suchmaterial. These drying trays 18 are provided with a plurality ofperforations or holes 28 arranged to allow the vapor conveying fluidwithin the cavity walls 14-16 to be exhausted upwardly from tray totray. To this end, the cavity walls 14 and 15 are also provided withperforations 30 and which latter perforations are arranged relative tothe drying trays 18 in horizontal alignment above the trays 18 to passthe vapor conveying fluid over the surface of the food pieces undergoingdrying. The size of the cavity wall perforations 30 is preferablyarranged to be small enough to block the passage of the electromagneticwaves while allowing the passage of a fluid therethrough. Theseperforations may be dimensioned' relative to the wall thickness topresent a short circuit to the electromagnetic radiation by anyconventional microwave technique.

Associated with the drying cabinet 11 isthe ultra-high frequencygenerator (UHF) 20. The UHF generator is connected to a suitable sourceof power 34 and capable of producing considerable alternating-currentpower at a frequency on the order of 1000 megacycles. The generator 20is connected through a wave guide 35 to the drying cabinet 11 andincludes a suitable coupling such as an antenna 36 projecting throughthe rear cavity wall 16 into the cavity 17 proper. In this fashion, thealternating voltage of the generator 20 is transmitted by means of theantenna 36 into the cavity 17, whereby centimeter electromagnetic Wavesare produced in the cavity 17 and which Waves comprise electric andmagnetic fields permeating completely the space in the cavity 17.

The heat generated by the UHF generator 20 is utilized in accordancewith this invention to preheat the vapor conveying fluid to be passedinto the drying cabinet 11. To this end the UHF generator 20-is shownenclosed in a fluid duct 37 having a fluid inlet portion 37a torereceive the atmospheric or ambient air. The air circulating in theduct 37 acts to cool the UHF generator 20 and thereby fis preheated Anoutlet portion 37b for the duct 37 conveys'the preheated fluid from thegenerator .20 to the heating means 21, wherein it is heated to its finaldrying cabinet temperature. The heating means 21 may comprise aconventional electric heater, such as a resistance wire heater,connected to a suitable source of power, shown by the block identied bythe reference character 38. The fluid received at the heating means 21is conveyed into the drying cabinet 11 by means of a conduit 39communicating with the heating means 21 and the drying cabinet 11. Theconduit 39 is connected to the bottom portion of the cabinet 11 throughthe metallic casing 12 to pass the air into the plenum chamber providedbetween the spaced walls of casing 12.

The exhaust means 22 for the drying apparatus 10 is supported on aplatform 40, suitably connected to the top of the drying cabinet 11. Theexhaust means 22 may be a conventional fan powered by a separate motor41, also connected to a separate power source 42. The exhaust means 22is utilized to exhaust the fluid or air from the drying cabinet 11 andmore particularly the cavity 17 therein. The exhaust fan is arrangedwithin the housing for exhaust means 22 so as to cooperate with anexhausting duct 43 communicating with the top of the drying cabinet 11and extending through the top wall 25. The exhaust duct 43 in thisfashion will convey the fluid from the drying cabinet 11 into theexhaust means 22, from which the fluid will pass out through the exhaustduct 44.

With the power source 42 connected to the fan for the exhaust means 22and the power source 38 connected to the heating means 21, the path ofthe fluid through the drying apparatus 10 may now be traced. The fluidwill enter the duct 37 by means of the inlet duct 37a, be passed aroundthe generator 20, then through the duct 37b into the heating means 21.The thus heated fluid will pass `from the heater 21 into duct 39 andthen into the drying cabinet 11. The fluid will be conveyed into theplenum chamber formed by the walls of the metallic casing 12 and willthen travel by means of the cavity wall perforations 30 into the cavity17 itself. With the exhaust means 22 operative at this time the pressureWithin the cavity 17 will have been reduced so as to draw this heatedair into the cavity 17 by means of the cavity wall perforations 30 touniformly distribute the fluid over the food pieces. As describedhereinabove, these cavity wall perforations 30 are arranged so as tocause the fluid passing therethrough to pass over the food pieces placedon the storage areas of the drying trays 18. Also, the fluid will beexhausted upwardly through the cavity 17 by means of the trayperforations 28. The fluid then will pass from the cabinet 11 throughthe conduit 43 and be exhausted by means of the exhaust conduit 44 fromthe drying apparatus 10.

Sensing means such as the temperature measuring apparatus 48 forcontrolling the temperature of the vapor convey-ing fluid passed intothe drying cabinet 11 may be provided to automatically control thetemperature of the fluid. When manual control of the fluid temperatureis desired, the apparatus 48 may be in the `form of a thermometer 1whichmay be inserted into the conduit 39 to measure the temperature of thefluid therein. Also, since this temperature control is dependent uponthe power absorbed by the food pieces lbeing dried, the power deliveredto the cavity 17 should also be observed by the operator of the dryingapparatus 10. This observation may be made by the provision of powerabsorption measuring apparatus 50 in the form of a meter suitablycorrnected to the drying cabinet 11 as the antenna 36 or any other wellknow manner. The desired temperature relationship and power loss for thedrying cabinet 11 can be better appreciated by examining FIG. 3. Theoperator, upon observing the power loss and the temperature of the vaporconveying fluid and correlating this information with a chart similar tothat of FIG. 3 for the food being dried, can adjust the heating means todecrease the fluid temperature. By way of example, the dryingtemperature in applications of the type to be described hereinafter maybe maintained at a temperature of 165 degrees Fahrenheit until the powerabsorbed in the cavity 17 has reached a point of approximately percent.After this initial drying interval, the operator should adjust theelectric heating means 21 to gradually rediuce the power deliveredthereto from the power source 38 so as to in turn gradually reduce thetemperature of the fluid conveyed to the drying cabinet 11. This fluidtemperature is gradually reduced until the point where the powerabsorbed in the cavity 17 is approximately l() percent, at which timethe fluid temperature should be 120 degrees Fahrenheit.

Having described the apparatus for drying the food pieces, the methodwill now be described more fully. The material or food pieces to bedried are first blanched, cleaned, and diced or divided into smallpieces or chunks of preferably bite size. The pieces should -be largeenough to be discrete pieces but small enoughl so that theelectromagnetic waves will completely penetrate them and heat themuniformly throughout. Such pieces of food may have their smallestdimension as much as two inches and can be of any convenient shape. Itwill also be appricated that the thickness of the food pieces is suchthat they will not be completely dried through in a very short time. Thefood pieces prepared in this fashion may then be placed on the dryingtrays 18 and in the instance where the trays 18 are removable from thedrying cabinet 11 the trays 18 are placed into the cabinet 11. Thecabinet door 13 is then closed and the drying operation may then bestarted by the operator. The exhaust means 22 is then powered from thepower source 42. The heating means 21 is also powered from itsassociated power source 38, and the UHF generator is also started. Itshould be noted at this point that it is desirable to match theimpendance of the generator 20 with that of the loaded cavity, and thismay be accomplished by a .suitable transformer and indicator asdescribed in the aibove identified Electronics publication.

After the UHF generator 20 has been energized, it will be effective totransmit the electromagnetic radiation to the drying cabinet 11 by meansof the waveguide 35 and the antenna 36. The electromagnetic radiationwill completely permeate the cavity 17 so that the food pieces thereinare heated entirely through at once so as to expel the natural fluidsdiffusing through the food pieces. As the iiuids are expelled from thesurface of the food pieces, the heated fluid entening the cavity will bepassed over the food pieces and which heated iiuid will absorb thefiuids and fluid vapors from the food pieces at about the same rate itis expelled from the food pieces and carry it out of the cavity 17 as aresult of the cooperating action of the exhausting means 22. Thiscoaction of the electromagnetic radiation and the heated iiuid is socontrolled in accordance with this invention to allow the food pieces todry completely and uniformly throughout without any hardening of theouter surface of the food pieces. Also, the electromagnetic radiation atthe preferred 1000 megacycle frequency will not result in the burning orheating of the food pieces so as to make them undesirable orunpalatable.

The superior product resulting from this drying operation will be a bitesize food product having a natural color and of low density. The foodproduct prepared in this rapid fashion is dried to a low fluid contentand may be stored for future use without deteriorating.

If it is desired to provide a product that is puffed or expanded withits attendant commercial advantages, it may be accomplished inaccordance with the procedure of this invention by increasing t-heelectromagnetic radiation delivered to the cavity 17 to cause the foodpieces to expel the uids therefrom at such a rate that the food pieceswill expand. In other words, the rate of evaporation within the foodpieces is greater than the rate of diffusion of the fluid vapor outwardthrough them to result in the increased volume of the pieces. Anexpanded or puffed food product prepared in this fashion will retain itspued appearance after the drying operation is completed.

It should be noted that an important feature of this invention is thatit is not necessary to provide an external sounce of steam to the foodto be dried for expanding the food. In conventional drying methods thesteam is required to provide the internal pressure for expanding orpuffing the food prior to the drying thereof.

When it is desired to produce a dried fruit piece for a lbreakfast food,a drying cycle of about one hour may be employed. In conventionalpresent day commercial drying apparatus the same fruit piece wouldrequire a 24-ho-ur drying cycle. This one-hour drying cycle results whenthe iiuid is passed through the cavity 17 at a rate of approximately 600feet per minute to dry fruit pieces having a size of 5%; inch x 5/8 inchx 5/s inch and having an initial li-uid content of about percent. Thefood pieces can be quickly dried by this invention to about 14 percentmoisture to produce sweet, unwrinkled, stable fruit pieces suitable fora breakfast food.

It will lbe recognized by those skilled in the drying art thatvariations and modifications may be made within the scope of thisinvention. Some of these changes may include changes in the shape of thecavity, automatic pro- .gramming of the fluid temperature for the dryingcabinet by sensing the power loss of the UHF generator and programmingthe fiuid temperature as a function of such power loss, such fiuidtemperature being changed by varying the power delivered to theelectrical heating means. Furthermore, the hrumidity of this fiuid andthe fluid velocity relative to the power delivered to the cavity may becontrolled. Also, such a method is compatible for drying food piecesbatchwise or adaptable for a continuous process.

What is claimed is:

1. Drying apparatus comprising a drying cabinet having anelectromagnetic cavity therein defined by perforated walls internallyspaced within said cabinet and including dielectric shelves supportedwithin said cavity, means for producing and propagating ultra-highfrequency electric and magnetic fields within said cavity, means forpassing a heated, dry -gas into the cabinet for carrying away the fluidfrom a material being dried through said cavity, and means forwithdrawing the fluid laden gas from said cavity.

2. Drying apparatus comprising a drying cabinet including a metalliccasing having a plurality of outer walls spaced from a plurality ofelectrically conducting perforated walls for defining an electromagneticcavity within the casing and a gas passage between said thus spacedwalls, a plurality of dielectric perforated trays supported within thecavity and adapted to receive food pieces to be dried, means forsupplying a heated, dry gas to the gas passage for the drying cabinet,said electrically conducting walls and said perforated trays beingarranged whereby a gas entering the cavity through the perforationscontacts the food pieces, means for producing and propagating ultra-highfrequency radiation within said cavity proportioned to dry the foodpieces placed on said trays, and means for withdrawing the gas from thecasing through said cavity.

3. Drying apparatus as defined in claim 2 including means forcontrolling the temperature of the gas supplied to said drying cabinet.

4. Drying apparatus including a drying cabinet having a gas passagedelined by imperforate walls spaced from a plurality of electricallyconducting perforated walls, the latter walls arranged and constructedto form an electromagnetic cavity within the cabinet, a plurality ofdielectric perforated trays slidably supported within the cavity toreceive food pieces to be dried, means for delivering a heated, dry gasinto the gas passage, said electrically conducting walls and saidperforated trays being perforated at preselected points -to therebyallow the gas to enter the cavity by means of same and in contact withthe food pieces, means for producing ultra-high frequency radiationwithin said cavity proportioned to be reflected from the conductingwalls to permeate completely the cavity of dry foods pieces on saidtrays by expelling the fiuids therefrom, and means for exhausing thefiuid laden gas from the cabinet through said cavity.

5. Drying apparatus as defined in claim 4 wherein the ultra-highfrequency radiation is in the centimeter range 7 and the perforationsfor the electrically conducting walls are dimensioned relative to thecentimeter length of the radiation to block the passage of the radiationtherethrough but allowing the passage of the fluid laden gas throughsame.

6. Drying apparatus as dened in claim 4 including a closure member forthe drying cabinet having an inner wall `cooperating with saidperforated walls to coinplete the cavity.

7. Drying apparatus as defined in claim 4 wherein the means forproducing the ultra-high frequency radiation includes an ultra-highfrequency generator and said means `for delivering the heated gasintothe passage includes means for passing the gas adjacent theultra-high frequency generator for preheating the gas and cooling thegenerator.

8 References Cited by the Examiner UNITED STATES PATENTS 10/1918 Harvey34-48 3/1924 Drake 34-88 4/ 1947 Birdseye 99- 204 7/ 1947 Peterson 34--17/1950 Bradbury 34-1 12/1950 Phillips 34-1 10/1952 Wade 34-1 4/1954Pound 34-1 4/1956 Thomas 34-1Y FOREIGN PATENTS 2/ 1949 Great Britain.

WlLLIAM F. ODEA, Acting Primary Examiner.,

NORMAN YUDKOFF, Examiner.

1. DRYING APPARATUS COMPRISING A DRYING CABINET HAVING ANELECTROMAGNETIC CAVITY THEREIN DEFINED BY PERFORATED WALLS INTERNALLYSPACED WITHIN SAID CABINET AND INCLUDING DIELECTRIC SHELVES SUPPORTEDWITHIN SAID CAVITY, MEANS FOR PRODUCING AND PROPAGATING ULTRA-HIGHFREQUENCY ELECTRIC AND MAGNETIC FIELDS WITHIN SAID CAVITY, MEANS FORPASSING A HEATED, DRY GAS INTO THE CABINET FOR CARRYING AWAY THE FLUIDFROM A MATERIAL BEING DRIED THROUGH SAID CAVITY, AND MEANS FORWITHDRAWING THE FLUID LADEN GAS FROM SAID CAVITY.